Nitride Compounds: Property Anomalies Near Structural Instabilities

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NON-TECHNICAL DESCRIPTION: This project is developing a new type of material consisting of a mixture of two different materials that are forced together. Naturally these two materials would not mix; however, if mixed artificially, the mixture exhibits unique properties because a small external stimulus can lead to a large change in the material structure. The unique properties anticipated for this material include a strong deformation when applying a voltage or a magnetic field. For example such a material is useful for building a new type of frequency filter, which would increase the data rate of mobile devices by ten to hundred times. These new properties may facilitate a whole range of transformative applications, including, for example, magnetically driven micro-switches for biomedical implants, emergency valves in nuclear power plants that operate without any external power source or communication pathway, tough and resilient coatings that remain functional despite frequent temperature cycling (for possible application in high-temperature fuel-efficient jet engines).TECHNICAL DETAILS: This project is developing an understanding of structural instabilities in ternary nitrides ABN where AN and BN form different equilibrium crystal structures. The key idea is that single-phase solid solutions of such ternaries with an intermediate composition exhibit a structural instability that leads to anomalous physical properties, including a reversible plastic deformation or a diverging piezoelectric response. Ternary nitride layers are deposited by reactive sputtering. Epitaxial constraints, composition variations, and ion-surface interactions are used to control the emerging phases, microstructure, texture, strain, and possible phase separation. The structural instabilities of single-phase solid-solution ternary nitrides at elevated temperature and pressure are studied to determine the structure-strain-energy landscape, using in situ x-ray diffraction, Raman and infrared spectroscopies. Emerging anomalous properties are measured and an understanding of how the structural instability leads to a reversible plastic deformation, an anisotropic effective Poisson's ratio, a divergence in the piezoelectric response, a magneto-mechanical coupling, and a reversible stress-induced shear transformation are developed. The project includes training of graduate and undergraduate students in state-of-the-art materials synthesis and characterization, as well as involvement in major outreach efforts to encourage girls to consider a career in science and engineering.